sts92_jetpack_001018 CAPE CANAVERAL, Fla. Talk about a worst-case scenario.
Laboring in a life-preserving spacesuit,
a spacewalking construction worker at the International Space Station (ISS) suddenly is cast adrift when braided steel safety tethers snap, sending the astronaut tumbling into the deadly vacuum of space.With no shuttle docked at the outpost, the towering station itself would be unable to give chase. Spacesuit oxygen supplies would be exhausted within a matter of hours, and the drifting spacewalker would be subjected to a certain, suffocating death.
In a bid to avoid that scary situation, astronauts aboard
shuttle Discovery set out Wednesday to test-fly small jet backpacks designed to give errant astronauts a fighting chance to zoom back to the station in an emergency. 
Spacewalkers Leroy Chiao and Bill McArthur work outside the International Space Station during the third extravehicular activity (EVA) of mission STS-92 on October 17, 2000.
Considering the staggering amount of spacewalking work still required to raise the station 156 more excursions are planned NASA officials say the jetpack test-flights are crucial.
"Its like wearing a parachute when you go flying. You have it, you know how to use it, but you just hope you never have to," NASA flight director Chuck Shaw said.
"The likelihood of coming free is pretty small," spacewalker Jeff Wisoff added in a preflight interview with SPACE.com. But "if youre going to depend on [the backpack] as a parachute, then you should know it works. And doing a test-flight is a good way to do that."
Wisoff and spacewalking partner Michael Lopez-Alegria started getting ready for the backpack test-flights early Wednesday, shortly after flight directors radioed up some wake-up music to the orbiting shuttle crew.
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Chosen by Wisoffs wife fellow NASA astronaut Tamara Jernigan the apt selection for the day was the theme from the television show Mission Impossible.
"Good morning, Jeff, and good luck on yet another 'Mission Impossible," European Space Agency astronaut Gerhard Thiele said from NASAs Mission Control Center in Houston, Texas.
Outfitted in $12 million spacesuits, Wisoff and Lopez-Alegria were set to float into the shuttles cargo bay just around 11 a.m. Eastern Daylight Time (15:00 GMT).
The first order of business: "Clearing the deck"
on the stations new rooftop truss, which was mounted to the 13-story outpost Saturday.That work will involve removing a grapple fixture that enabled Japanese astronaut Koichi Wakata to latch onto the truss with the shuttles 50-foot (15-meter) robot arm and put it in place on one of three permanent modules that now make up the station.
The Z-1 truss, with its large white antenna dish, is seen attached to the ISS as Discovery's robot arm pulls away on October 14, 2000 during mission STS-92.
Shaped like a round dish with a large pin protruding from it, the fixture must be detached and stowed to make way for a pair of power-producing solar arrays to be delivered
aboard shuttle Endeavour after a planned November 30 launch.The 9-ton truss will serve as a temporary mounting point for the giant solar panels, which will have a wingspan of 240 feet (73 meters) once unfurled in orbit.
The spacewalkers also will swing open a square cable tray on the truss so that a future assembly crew can hook coolant lines to it. The ammonia lines will connect to radiators on the solar arrays, providing a means to shed heat generated by station electronics.
The deployment of the cable tray will set the stage for the jetpack tests.
Anchored at the end of the shuttles robot arm, the spacewalkers will be hoisted one at a time to a point about 50 feet (15 meters) above the shuttles cargo bay.
A highly choreographed test-flight regime first calls for the astronauts to see how well the 85-pound (38.5-kilogram) jetpacks can be steered. Powered by 24 nitrogen gas thrusters, the units are controlled with hand-held joysticks.
In a close approximation of a real emergency, the astronauts then will try to fly straight toward a TV camera mounted in the rear bulkhead of the shuttles cargo bay.
"Its going to be a very slow, deliberate process of getting down there," said Daryl Schuck, a lead engineer in NASAs spacewalk projects office at Johnson Space Center.
"Itll probably take on the order of three to four minutes to get to actually get to the aft bulkhead. So its not going to be a very fast process. Its going to be very controlled, and very deliberate, and thats how an actual rescue would happen."
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Unlike a real emergency, though, the astronauts will remain tethered to the robot arm during the jetpack flights. Thats because Discovery could not quickly undock from the station and chase after the astronauts if something went wrong during the test.
Said Schuck: "We obviously do not want to take the risk of having a crew come loose during just a demonstration test like this when we actually couldnt execute a rescue."
Wisoff and Lopez-Alegria also will stage a second emergency drill to see if a spacewalking construction worker at the station could carry an unconscious partner back to the shuttle for medical attention in an emergency.
"We call it the 'dead-guy test," said Schuck.
The astronauts will take turns "playing dead," or acting as if they are suffering from an orbital version of "the bends" the type of decompression sickness that can make scuba diving deadly.
The idea then is to haul the "incapacitated" spacewalker back to the shuttles airlock just to make sure it could be done in a real emergency.
"You can have the same type of decompression sickness on orbit doing a spacewalk, and if we ever had a crew member have that happen to them while theyre outside, they might go unconscious," Wisoff told SPACE.com before Discoverys launch.
"So we want to make sure its a reasonable expectation that one crewmember can drag the other person into the airlock so we can get him some medical attention if necessary."
The planned 6.5-hour excursion will be the fourth and final spacewalk of
NASAs 100th shuttle flight, considered by many to be the agencys most ambitious station construction mission to date.The six-man, one-woman crew plans to spend the day Thursday inside the station, delivering supplies for the outposts first full-time tenants, who are due to take up residence at the station in early November.
The astronauts are to depart the station Friday, heading out on a two-day trip back to Earth. Landing at Kennedy Space Center is scheduled at 2:10 p.m. EDT (18:10 GMT) Sunday.
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